Thanks - fixing the must -> shall (like the illegal -> invalid) is something for MPI-next. There are currently are large number of "must"s in the document. Bill William Gropp Director, Parallel Computing Institute Deputy Director for Research Institute for Advanced Computing Applications and Technologies Paul and Cynthia Saylor Professor of Computer Science University of Illinois Urbana-Champaign On Aug 26, 2012, at 11:47 AM, N.M. Maclaren wrote:

On Aug 26 2012, Rolf Rabenseifner wrote:

...

About 1.: Yes, agreed. Always must and should are used as synonyms in the scope of standards. To use the same words in both sentences is clarifying that there is no relevant difference in the meaning.

Sorry, but definitely not in ISO standards! From ISO Directives Part 2:

Table H.1 - Requirement Verbal form Equivalent expressions shall is to is required to it is required that has to only ... is permitted it is necessary shall not is not allowed [permitted] [acceptable] [permissible] is required to be not is required that ... be not is not to be

Do not use "must" as an alternative for "shall". (This will avoid any confusion between the requirements of a document and external statutory obligations.) Do not use "may not" instead of "shall not" to express a prohibition.

Table H.2 - Recommendation Verbal form Equivalent expressions should it is recommended that ought to should not it is not recommended that ought not to

[End of extract]

However, in non-ISO standards (and fairly often even in them!, "must" is used as a synonym for "shall". It is definitely never a synonym for "should".

Regards, Nick Maclaren.

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